The Federal Communications Commission (FCC) has recently ruled that wireless communications devices are no longer exempt from the hearing aid compatibility magnetic coupling requirements of the Americans with Disabilities Act. This requires that a wireless communciations device produce a magnetic field with a specified strength and frequency response which can be detected by the telecoil of a hearing aid. This is in addition to the requirement that a wireless communications device produce an audio signal having a specified strength and frequency response which can be detected by the microphone of a hearing aid.
Traditional desktop telephone handsets use dynamic receivers that, with minimal acoustic design, are able to provide a nominally flat audio frequency response as required by the relevant specifications. The inherently flat magnetic field generated by the receiver also makes it relatively easy to meet the required magnetic frequency response. If the level of the magnetic field is insufficient (or completely absent as with piezoelectric receivers) then an additional device telecoil may be used as is commonly known to those skilled in the art. This additional telecoil may be included as an integral part of the receiver or implemented separately.
However, the smaller size of the newer wireless communications devices and the acoustic design changes that are necessary for good audio performance frequently make it difficult and sometimes impossible to simultaneously meet both the audio and magnetic specifications (particularly the frequency response) even with an additional telecoil. Frequently, the wireless communications device must rely on equalization (EQ) of the electrical signal that corresponds to the output audio signal, usually using a digital signal processor (DSP), to meet the audio frequency response requirements. This same EQ directly affects the signal creating the magnetic field and may cause the wireless communications device to fail the magnetic frequency response requirements. This is the case regardless of whether or not there is an additional telecoil because the same drive signal is used to drive both the speaker and the additional telecoil.